U.S. Pat. No. 3,038,439, issued June 12, 1962 to Martin et al, discloses a relay control system which is particularly applicable for protecting and controlling the operation of internal combustion engines such as heavy diesel engines. The control system disclosed in the patent incorporates a plurality of indicating relays which are operated pneumatically. Each indicating relay is operatively associated with a sensing device such as normally closed two- or three-way valves which detect undesirable conditions such as temperatures, pressures, vibration, liquid levels, and so forth.
A sensing device is connected to a relay trip port and pressurized fluid is introduced into an input port of the relay. As long as the sensed condition is normal the relay directs the pressurized fluid entering the input port through an output port, the output port, in most cases, being in communication with the input port of a relay of like construction in the control system.
When, however, a condition is sensed by the sensing device which falls outside desired perimeters, the relay will operate to cut off the input port from the output port and place the output port in communication with a vent port. This will result in venting of the residual pressure in the output port and tripping of all the down stream relays.
In the arrangement of U.S. Pat. No. 3,038,439, the operation is carried out by means of a valve assembly in the interior of the relay casing, said valve assembly being generally in the form of a piston movable within the interior of the casing.
With the arrangement of U.S. Pat. No. 3,039,439, a problem can present itself in that the valve assembly or piston mounted within the casing can "bounce back" from its tripped position if for some reason the pressure at the input port of the indicator relay falls below a predetermined minimum. This undesirable action results from residual pressure in the line extending from the trip port. In essence, the trip signal line pressure can result in the resetting of the indicating relay to its normal operative position when such is not desired, thus creating a condition which can cause damage to the engine or other device being sensed.